The relaxing effect of caerulein on isolated human internal anal sphincter

In the attempt to find a pharmacological treatment for the spasm of the internal anal sphincter, usually associated with anal fissures, the activity of caerulein on human internal and sphincter was investigated in vitro and in vivo. In the isolated distal part of the internal and sphincter, caerulein (0.61 microM) depressed resting muscle tone and caused marked relaxation of norepinephrine-contracted preparations. The effect of caerulein was reduced by atropine and increased by physostigmine, suggesting that it was largely due to the release of acetylcholine. In vivo, intravenous infusion of caerulein, both to healthy volunteers and to subjects affected by anal fissures and anal sphincter hypertone, did not modify the values of internal anal sphincter pressure. The lack of spasmolytic effect of caerulein in vivo may have been due to the relatively unimportant influence of cholinergic neurons on the control of internal anal sphincter tone. Alternatively, the presence of fibrosis caused by anal fissures could hinder sphincter relaxation.